Insulating, dielectric fluids are used in electrical apparatuses like transformers, capacitors, switchgear, bushings, etc., and have a multitude of functions. Dielectric fluids act as electrically insulating medium separating the high voltage and the grounded parts within the apparatus and function as a cooling medium to transfer the heat generated in the current-carrying conductors. Additionally, the fluids provide a medium to monitor the health of a transformer during operation.
In addition to the basic abovementioned functions, the insulating liquid should also comply with other necessary and desired requirements. The fluid should have a high efficiency, long life, and minimal environmental impact. Further, the fluid has to be compatible with the materials used in the electrical equipment and it should not constitute a hazard for the health and safety of personnel. In practice, insulating fluids should fulfil various physical, electrical, and chemical properties and all these properties are regulated through standards and specifications that stipulate the minimum requirements for each one of the important properties.
Traditionally, petroleum-based oils have been used as the insulating fluid in oil-filled transformers mainly because of advantageous properties relating to low viscosity, low pour point, high dielectric strength, easy availability and low cost. During the last couple of decades, the transformer industry has been undergoing several changes. The market demand for compact and efficient transformers with guaranteed long-term performance coupled with the problems of corrosive sulphur and oil quality issues have warranted the need for enhancement in the properties of transformer oil. Further, strict environmental regulations towards health and safety have been steadily evolving and the huge liability risks in the case of transformer fires or outages have raised a cause for concern. Considering these factors, serious research and development efforts have since the 1990 been directed towards identifying alternatives to mineral oil.
Amongst the several options which are generally known, e.g., ester-based fluids, silicone fluid, chlorinated benzenes, perchloroethylene, polyalphaolefins etc., ester based fluids (both synthetic and natural) are excellent alternatives to mineral oil, primarily due to their high biodegradability (lower environmental risk) and high values of flash points and fire points (high fire safety factor). Further, natural esters based on vegetable oils, with the main constituent being triglycerides, are preferred due to their renewability.
There are consequently substantial needs in the art for improving the performance of ester-based fluids, and more specifically triglyceride-based fluids, for power and/or electrical applications, in order to replace the rather disadvantageous insulation fluids currently utilized within the industry.
Generally, all vegetable oils have a high viscosity as compared to mineral oil. If a transformer has to be operated at higher voltage levels, it may occasionally be necessary to circulate the oil inside the transformer through pumps. The high viscosity of vegetable-based liquids then poses several challenges towards the design of the transformer, especially from a cooling point of view. This leads to the requirement of a lower viscosity value for vegetable-based fluids.
Biodegradable natural ester-based fluids have high pour point temperatures as compared to mineral oil, which can be considered as a major drawback if the electrical apparatuses comprising the fluid have to be operated in extremely cold environments, a problem that is especially pronounced at higher voltage ratings. Further, a low pour point can cause changes in the dielectric and/or other properties of the fluid and the solid insulation impregnated with this fluid. This in turn can force design changes in the transformer which can lead to an increase in the manufacturing costs. A very low value of pour point is therefore also desired for the vegetable fluid.
For performing the electrical insulation function, the insulating fluid must be designed to withstand the required electrical stresses as per the design specifications of the electrical apparatus.
Electrical streamers are pre-breakdown phenomena in the form of low-density conductive structures that form in regions of fluid that are over-stressed by electric fields on the order of 1×108 (V/m) or greater. Once a streamer forms it tends to elongate, growing from the point of initiation towards a grounding point. The extent of a streamer's development depends upon the nature of the electrical excitation which caused it. Sustained over-excitation can result in a streamer bridging the fluid gap between its point of origin and ground. When this happens an arc will form and electrical breakdown will occur. Streamers can form due to both positive and negative excitations (Sullivan, Thesis (Ph. D.), Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007).
The dielectric breakdown withstand voltage under AC (50/60 Hz) and Lightning Impulse (1.2/50 μs) is considered as the most important parameter from an electrical insulation perspective. The dielectric breakdown withstand voltage (breakdown voltage) can be defined as the voltage required to obtain a flashover in the oil between two electrodes of specified shape and placed at a certain distance from each other. The AC voltage is the line frequency of the mains (either 50 or 60 Hz depending on where you live). The lightning impulse (LI) breakdown voltage is simulating lightning strikes, and usually uses a 1.2 microsecond rise for the wave to reach 90% amplitude then drops back down to 50% amplitude after 50 microseconds. Two technical standards governing how to perform these tests are ASTM D1816 (mainly for AC) and ASTM D3300 (for impulse voltages). The standards specify the type of electrodes and the gap distances required for the tests.
One of the parameters associated with the lightning impulse (LI) breakdown phenomenais the speed at which a streamer propagates from the initiation point to the ground. An important parameter with respect to LI streamer speeds is the acceleration voltage (Va), which can be defined as the voltage at which the speed of the LI streamers accelerates to a very high value.
FIG. 1 generally illustrates a difference in streamer velocity between a natural ester dielectric liquid and mineral oil. The natural ester has an average breakdown voltage (Vb) of about 140 kV, beyond which the speed of the streamer is observed to accelerate sharply. So, practically, Va coincides with Vb in the case of ester liquids, i.e. the ratio of Va/Vb is close to 1. On the other hand, in the case of mineral oil, the ratio of Va/Vb is around 1.5 which is much higher. In addition, the breakdown voltage of mineral oil is also higher as compared to the ester liquid.
For a high safety factor in the electrical apparatus, it is always desirable to have a slow streamer speed, i.e. a high breakdown voltage and a higher ratio of Va/Vb. In this respect, ester fluids do not perform similar to traditional mineral oils. Ester dielectric fluids generally have fast LI streamer speeds, typically above 100 km/s (Duy, et al., IEEE Transactions on Dielectrics and Electrical Insulation, 2009, Vol. 16, 6, pp. 1582-1594, and Rongsheng L. et al., IEEE Conference on Electrical Insulation and Dielectric Phenomena, (CEIDP) 2009, 18-21 Oct. 543-548, ISSN: 0084-9162). Therefore, special caution is required in the design of electrical apparatus with ester fluids.
It is known in the art to improve the properties of ester oils used in transformers by the addition of additives. Common additives used for ester oils are anti-oxidants, pour point depressants and metal passivators (see for example U.S. Pat. No. 6,274,067).
Further, in the international patent application WO 2008/071704, an insulation liquid for electrical or electromagnetic devices is disclosed, wherein the liquid comprises a carrier liquid and nano-particles. The nanoparticles preferably have a conductivity of 10−5 to 105 S/cm in order to reduce the streamer speed of a positive streamer.
Also the US patent application US 2011/232940 discloses an insulating liquid that includes an ester liquid and an additive to the ester liquid, whereby a reduction in the formation of fast electrical streamers is allegedly obtained. However, no experimental data is provided supporting this.
There are substantial needs in the art for improving the LI steamer speeds of ester-based dielectric fluids in order to enhance the safety and performance of electrical apparatus used with ester-based dielectric fluids.